The production of crosslinked, highly elastic synthetic resins by the reaction of substantially linear hydroxyl polyesters or hydroxyl polyethers with an excess of 1,5-naphthylene diisocyanate, followed by reaction with chain lengthening agents has long been known. In the first stage of this process, the polyester or polyether chains are linked via urethane groups. Substantially linear prepolymers which contain free isocyanate groups at the ends of the chains are obtained. The smaller the quantity of diisocyanate in excess of that required for a complete reaction with the hydroxyl groups present, the higher is the molecular weight of these prepolymers. The isocyanate prepolymers obtained in this way can be converted into high quality crosslinked polyurethane elastomers by two principal processes.
In one process described in German Patent Specification No. 831,772, the isocyanate prepolymers are reacted with slightly less than the equivalent quantity of glycols. The first reaction to take place is a chain lengthening of the prepolymers via urethane groups. In the second phase, the excess isocyanate groups react with the hydrogen atom of the urethane groups to form allophanate groups and thus cause crosslinking of the molecule.
The second principal process consists of a reaction of the isocyanate prepolymers with slightly less than the equivalent quantity of water or diamines. The prepolymers are chain lengthened via urea groups. The hydrogen atoms of the urea groups react in a second phase with excess isocyanate groups. This results in crosslinking, with the formation of biuret groups. Since biuret groups are thermally more stable than the allophanate groups formed in the first mentioned process, elastomers produced using water or diamines as the chain lengthening agent are superior in their mechanical properties, and in particular relative to their structural stability, elasticity, pressure deformation resistance and abrasion properties. In order to prevent the development of a cellular structure in the elastomer due to the liberation of carbon dioxide, which takes place when isocyanates react with water, the material may be molded under pressure.
When isocyanate prepolymers are crosslinked with glycols or water on a large commercial scale, it is absolutely necessary for the hot reaction mixtures to have both sufficiently long pouring times and short setting times (mold release times). The possibility of rapid removal of the elastomers from their molds is essential for achieving short operating cycles and hence optimizing the process economically. It is, therefore, generally necessary to accelerate the chain lengthening reaction in some manner.
It is known that the reaction between isocyanate groups and hydroxyl groups or water can be accelerated by various catalysts, such as tertiary amines, phenolates, alcoholates or organometallic compounds. However, all of the catalysts used in practice have serious disadvantages when used with 1,5-naphthylene diisocyanate. Due to the high melting point of 1,5-naphthylene diisocyanate (127.degree. C.), relatively high operating temperatures (about 110.degree. to 120.degree. C.) must be used both during the preparation of the prepolymer and during the chain lengthening reaction. The usual polyurethane catalysts give rise at these temperatures to undesirable side reactions, such as trimerization of the isocyanate groups or excessive allophanatization or biuretization. The end products are, therefore, in many cases too highly crosslinked, with the result that they have poor mechanical properties, and in particular inadequate tear resistance. Furthermore, although these catalysts shorten the mold release time of the polyurethanes, the pouring time of the reaction mixtures is also drastically reduced. Due to the rapid increase in viscosity of the reaction mixture, controlled working up is no longer possible, especially in the case of large reaction batches. Moreover, adequate flow of the reaction mixture to fill every part of the mold can not be ensured, especially if the mold has a complicated shape.
At the high temperatures mentioned above, the tertiary amines and organic metal compounds most commonly used as accelerators in polyurethane chemistry accelerate the oxidation of the reactants by atmospheric oxygen. In addition, amines frequently produce troublesome discoloration and impart an unpleasant odor to the synthetic resin end product. Amines and organo-metallic compounds accelerate the saponification of those reactants which are capable of undergoing hydrolysis, so that these reactants, particularly the polyester polyols frequently used in practice, readily undergo hydrolytic degradation.
It has now surprisingly been found that N-alkyl ureas are excellent catalysts for the reaction of 1,5-naphthylene diisocyanate with polyols or for the chain lengthening reaction of prepolymers based on 1,5-naphthylene diisocyanate and do not have the disadvantages of the usual catalysts described above. The pouring time of the reaction mixtures is not sufficiently reduced by the N-alkyl ureas in spite of the high reaction temperature. The mold release time of the elastomers is substantially shortened. N-alkyl ureas do not impair the stability in storage of isocyanate prepolymers based on 1,5-naphthylene diisocyanate even at temperatures of about 100.degree. C., in contrast to the usual catalysts which cause considerable trimerization and allophanatization during storage of the prepolymer.
N-alkylureas have already been described in British Pat. No. 1,463,809 as activators for the reaction of isocyanates with polyols. The isocyanates mentioned in this connection are tolylene diisocyanate and diphenylmethane diisocyanate. The process according to the invention differs in this respect in that it uses 1,5-naphthylene diisocyanates. It was found that N-alkyl ureas have a highly selective action for this isocyanate. When prepolymers based on 1,5-naphthylene diisocyanate are compared with prepolymers based on 4,4'-diisocyanatodiphenylmethane, each activated by the addition of N-methylurea, it is found that the reaction mixtures in the case of diisocyanatodiphenylmethane have insufficient pouring times for practical purposes and the mold release time is increased by a factor of about 2.5 (see Examples).
German Offenlegungsschrift No. 1,694,249 relates to the preparation of polyurethane foams based on polyether polyols, polyisocyanates, water and/or other blowing agents in which the foaming reaction takes place in the presence of open or cyclic esters of hexavalent sulphur. In this process, a wide variety of N-substituted ureas may be used as catalysts for the foaming reaction. The process according to the invention differs from this process, firstly in that the operating conditions employed according to the invention are such that substantially homogeneous elastomers are obtained; and secondly in the use of naphthylene diisocyanate as the isocyanate component; and lastly, in that, in the process according to the invention, no other catalysts are used.